The present invention relates to a flat image display device in which electrons emitted from a plurality of cathodes disposed on a cathode board impinge on a phosphor screen coated on an inner surface of a front glass case to display an image.
FIG. 12 is a cross-sectional view schematically showing a conventional flat image display device. As shown in FIG. 12, the conventional image display device comprises a front glass case 31 having a phosphor screen 32 on an inner surface thereof and a rear case 33. The front glass case 31 and the rear case 33 are hermetically sealed by frit glass at a sealing portion 35. Within an airtight chamber 34 are provided a cathode board 36 having cathodes facing the phosphor screen 32 for emitting electrons and a collector electrode 37 for collecting electrons emitted from the cathodes. As shown in FIG. 12, the cathode board 36 is supported by a plurality of support columns 38 fixed to the inner surface of the rear case 33 to face the phosphor screen 32.
FIG. 13 is an enlarged cross sectional view schematically showing a broken line part 40 of FIG. 12. In FIG. 13, a reference numeral 41 denotes a cathode (for instance, a conical cathode) for emitting electrons. A plurality of cathodes are orderly arranged in matrix form and corresponds to phosphor dots composing the phosphor surface 32. In FIG. 13, a reference numeral 42 denotes a cathode electrode for applying a voltage to the cathodes 41, a reference numeral 43 denotes an insulating layer, and a reference numeral 44 denotes a gate electrode.
In the above-described image display device, the electrons are emitted from the desired cathodes 41 when a predetermined negative voltage is applied to the cathode electrode 42 and a predetermined positive voltage is applied to the gate electrode 44. The emitted electrons are converged by electrostatic lens effect of the penetrating hole 37a formed in the collector electrode 37, and impinge on a metal back layer (not shown) provided on the phosphor surface 32 and to which a high voltage (e.g., +10 kV) is applied. As a result, the phosphor dots of the phosphor screen 32 emit light to form an image.
However, in the above-described conventional image display device, since the cathode board 36 is supported by the support columns 38 fixed to the rear case 33, a deformation or inward warp of the rear case 33 occurring after ejection of gas from the airtight chamber 34 causes a deformation or warp of the cathode board 36 toward the phosphor screen 32. As a result, a positional relationship between the cathodes 41 of the cathode board 36 and the phosphor dots of the phosphor screen 32 is changed, so the electrons emitted from the cathode electrodes 41 cannot impinge on the adequate phosphor dots, making it impossible to form an image of high quality.
Further, in the above-described conventional image display device, wiring of the lead lines 39 for applying the drive voltage to the cathodes 41 of the cathode board 36 is performed so that the lead lines 39 extend from the cathode board 36 through the sealing portion 35 to outside the case, while maintaining the insulating performance between the respective lead lines. This makes assembling the image display device very difficult.
Furthermore, in the above-described conventional image display device, since the outer and inner surfaces of the face portion 31a of the frond glass case 31 are flat, the face portion must be made thick in order to resist external atmospheric pressure. This, however, has caused a problem that an image is perceived as being floated near the edges of the face portion 31a and a displayed image is perceived concavely.